In imaging systems commonly used today, a charge retentive surface is typically charged to a uniform potential and thereafter exposed to a light source to thereby selectively discharge the charge retentive surface to form a latent electrostatic image thereon. The image may comprise either the discharged portions or the charged portions of the charge retentive surface. The light source may comprise any well known device such as a light lens scanning system or a laser beam. Subsequently, the electrostatic latent image on the charge retentive surface is rendered visible by developing the image with developer powder referred to in the art as toner. The most common development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the charge retentive surface to form a powder image thereon. This toner image may be subsequently transferred to a support surface, such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both.
In order to fix or fuse the toner material onto a support member or substrate, such as a sheet, permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with a material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retrained by the heated fuser roller and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate. In order to prevent this from happening, a release agent application mechanism is generally utilized.
Wide, small diameter roll fusers also inherently suffer from excessive fuser and pressure roll deflection. The load on the fuser rolls required is a function of speed and type of image to be fused. Bending of a beam, or roller, is inversely proportional to the cube of the length thus, as fuser get wider the rolls bend appreciably more at a given load. Likewise, the bending of a beam with a round cross section, or roller, is directly proportional to the cube of the roll radius. So if it is desired to make the roll a little smaller the deflection increases significantly. The goal in a fuser nip is to produce nearly uniform load across the width. As the roll deflects the load at the ends increase thereby producing paper handing problems, if the load is too nonuniform (e.g. wrinkling or creasing of the sheet).
It is known that skewing the fuser roll with respect to the pressure roll and wrapping one roll around the other tend to counteract the uneven load distribution caused by roll bending. However, the bent shape roll is a curve which is a cubic function, and it is being wrapped around a circular roll with is a squared function. Thus, resulting load distribution is a maximum about one quarter of the roll length in from each end to get a "bow tie" nip. Skewing has been successfully employed for fairly stiff systems and very flexible systems. The former needs very little compensation and thus little "bow tie" effect is apparent while the latter requires a lot of skew but the stiffness is low enough that the "bow tie" effect is not visible. Skewing also generates lateral thrust forces that wear the roll surface.
It is also known to profile fuser rollers so that the shapes of the fuser tends to overcome the bending problem or even to place a third roll in pressure engagement with the fuser opposite the pressure roll to overcome the deflection in the ends of the fuser roll. Uneven roll load distribution can also be prevented by crowning one of the two fuser rolls. However, crowning of one of two fuser rolls results in nip velocity problems which induce paper wrinkle.
The following references may be of relevance to the present invention:
U.S. Pat. No. 3,941,558; Patentee: Takiguchi; Issued: Mar. 2, 1976. PA1 U.S. Pat. No. 4,393,804; Patentee: Nygard et al; Issued: Jul. 19, 1983. PA1 U.S. Pat. No. 4,557,588; Patentee: Tomosada; Issued: Dec. 10, 1985. PA1 U.S. Pat. No. 4,939,552; Patentee: Nakanishi; Issued: Jul. 3, 1990. PA1 U.S. Pat. No. 5,045,890; Patentee: DeBolt et al.; Issued: Sep. 3, 1991. PA1 U.S. Pat. No. 5,049,944; Patentee: DeBolt et al.; Issued: Sep. 17, 1991. PA1 JPPN-58-184173 (A); Patentee: Katou; Published: Oct. 27, 1983. PA1 JPPN-61-251881 (A) Patentee: Takizawa; Published: Nov. 8, 1986.
The foregoing references may be summarized as follows:
U.S. Pat. No. 3,941,558 discloses a rolled web impregnated with silicone oil for preventing offset. The web has a thickness of 2 mm, a total length of 50 cm, and travels 1 cm per thousand copies between the supply and take-up rollers. This system transfers about 0.003 cc of oil to the fuser per copy.
U.S. Pat. No. 4,393,804 discloses a rolled web system that moves between a supply core and take-up roller. A felt applicator supplies oil from a supply reservoir to the web. The take-up core is driven by a slip clutch at a speed greater than the speed of the pressure roller, thus exerting tension on the web. The web is between one and two mm in thickness and moves at a constant speed of 5 cm per 200 to 1,000 copies.
U.S. Pat. No. 4,557,588 discloses an image forming apparatus such as an electrophotographic copier, microfilm equipment, recording equipment, facsimile or printer. A movable cleaning member is maintained in contact with a member to clean its surface and the movement of the cleaning member is variably controlled according to the state of the image formation.
U.S. Pat. No. 4,939,552 discloses a cleaning device for cleaning the surface of an element of a copying apparatus to be cleaned by contacting a cleaning web therewith is connected to a driving mechanism of a scanning member of the coping apparatus and is driven in correlative movement with the scanning member. It is driven only when the scanning member returns to its original position by a one way clutch and by a constant amount of movement irrespective of the amount of movement of the scanning member.
U.S. Pat. No. 5,045,890 discloses a fuser apparatus for applying offset preventing liquid to a fuser roll including: a supply core; a rotatable take-up core; an oil impregnated web member adapted to be moved from the supply core to the take up core; a motor mechanically coupled to the take up roll for driving the web member from the the supply core to the take up core; a pressure roll in engagement with the web member and positioned to provide a contact nip for the web member with the fuser roll opposite the pressure roll wherein the contact of the web member with the fuser roll transfers oil from the web member to the fuser roll, and control means to vary the duty cycle operation of the motor to drive the web member at a relatively constant linear speed at the contact nip, the control means including a timer to monitor the cumulative time of operation of the motor and means to progressively decrease the duty cycle of the motor in response to the cumulative time of operation wherein the progressively decreased duty cycle of operation compensates for the increasing radius of the web member on the take up roll to maintain said relatively constant linear speed at the contact nip.
U.S. Pat. No. 5,049,944 discloses apparatus for applying offset preventing liquid to a fuser roll including an oil impregnated web to be moved relative to a fuser roll. A timer is employed to monitor the cumulative time of operation of a motor used to drive the web relative to the fuser roll and to progressively decrease the cycle of the motor so that essentially a uniform amount of the web is moved at each cycle.
JPPN-58-184173 (A) discloses a fuser apparatus in which one of the rolls is crowned in the center and the other is flared toward one end. The flared roller is also mounted at an angle to the center line of the crowned roller.
JPPN-61-251881 (A) discloses a fixed roller having an inverted crow shape along its length, which engages a belt entrained about two crown shaped rollers to act a part of a fixing device.
Additionally, there are several automatic printing machines commercially available, such as the Xerox 5028 model copier, which employ webs for providing release agents to fuser rolls. Other examples of such commercial devices, presently or currently available, include the Canon model 3225, 3725, 3000 series, 4000 series and 5000 series products. These products also all have liquid release agent impregnated webs supported between a supply roll and a take-up roll and urged into contact with the fuser roll by an open celled foam pinch roll.
In accordance with one aspect of the invention there is provided apparatus for fusing toner images to substrates, The apparatus comprises an elongated heated fuser roll having a crowned surface an elongated pressure roll having a crowned surface, with the pressure roll being supported for pressure engagement with the fuser roll to form a nip therebetween adapted to receive substrates. The apparatus according to this aspect can further comprise means for applying substantially uniform pressure and velocity on substrates in the nip, wherein the applying means comprises loading means for deflecting the pressure roll into pressure engagement with the fuser roll to form the nip. Means for supporting the pressure roll rotatably in contact with the fuser roll may also be supplied. The crowned surface of the fuser roll and pressure roll is approximately equal to the sum of the deflection of the fuser roll and the pressure roll. The pressure roll of this aspect of the invention can be provided with a surface having a maximum diameter in the central region thereof, and the fuser roll can be provided with a surface having a maximum diameter offset from the maximum diameter of the pressure roll. The fuser roll's surface can be provided with a maximum diameter positioned approximately one third of the length from one end thereof.
According to another aspect of the invention, there is provided a method for fusing toner images to substrates which comprises the steps of providing an elongated heated fuser roll having a crowned surface, and supporting an elongated pressure roll having a crowned surface in pressure engagement with said heated fuser roll to form a nip therebetween adapted to receive substrates. The method of this aspect can further comprise applying substantially uniform pressure in the nip and effecting substantially uniform velocity on substrates in the nip. The applying step can include deflecting the pressure roll and fuser roll in the nip. This aspect of the invention can further include fabricating the crowned surface of the pressure roll and the crowned surface of the fuser roll to correspond to the deflection of the fuser roll and the pressure roll induced by the applying step, and the method can comprise the step of transporting the substrate through the nip. Additionally, the method of this aspect may include the steps of heating the fuser roll so as to fix toner on the transported substrates thereto and offsetting the maximum diameter of the fuser roll from the maximum diameter of the pressure roll so as to apply a substantially uniform pressure.